Gas chromatography (GC) is an established analytical technique used for the separation, qualification and quantification of a broad range of volatile compounds that are not susceptible to decomposition upon vaporization. Relative retention times can be used to identify specific analytes in a crude sample or mixture of compounds provided the method conditions are constant and the retention time of the analyte of interest is known under the same set of conditions. A number of detectors may be used in GC, however, some GCs are connected to a Mass Spectrometer, which acts as the detector.
Mass Spectrometry (MS) is an analytical tool that measures the mass-to-charge ratio of charged particles. The MS technique for the analysis of compounds involves ionization of chemical compounds in a sample to generate high-energy charged parent molecules and fragments thereof. MS is commonly used for the qualitative analysis of organic compounds. MS can be used for the elucidation of compounds by manual interpretation of the resulting ion fragmentation pattern, which is unique to a specific compound under a given set of conditions, or by comparison to a mass spectral library of known compounds, including peptides.
Currently, sample identification using the gas chromatography-mass spectrometry interface (GC-MS or GC/MS) is predominantly based on the use of 70 eV electron ionization (EI) mass spectral libraries. In GC-MS, the sample is separated by the GC component into constituent analytes, which are then individually detected by the mass spectrometer. A mass spectrum is generated for each analyte in the sample mixture and is used for the identification of compounds in the mixture. Library-based sample identification is performed by comparing the experimental mass spectrum to all the library mass spectra and, then, the provision of a possible list of candidates for the sample identity with reducing order of fitting or of a matching parameter. Sample identification with MS libraries is, thus, predominantly based on fragment ions that provide a compound specific fingerprint. The structure of a specific molecule is elucidated through a set of fragment masses recorded by a detector and represented by a mass spectrum. Interpretation of the mass spectrum can be accomplished in several ways e.g., by comparison of the mass spectrum against a mass spectral library or by accurate mass.
MS libraries are both powerful and easy to identify compounds. However, sample identification with MS libraries has three major limitations: (1) given the millions of possible compounds, the libraries cannot be completely comprehensive; (2) a library can fail in sample identification because the sample is not included in the library, due to co-elution of two or more compounds or due to statistical errors; and (3) about 30% of sample compounds do not show a significant molecular ion in their 70 eV electron ionization MS. Thus, for some compounds, sample identification through libraries alone is not completely reliable due to the possibility of false identification of a similar compound or a degradation product.
Mass spectral sample identification achieved by measuring accurate mass typically involves mass measurement precision of a few parts per million, followed by computer based conversion of that accurate mass into a list of potential elemental formulas, which are arranged in order of increased deviation from the measured mass. For such inversion of experimental data into an elemental formula, the user must provide as an initial input parameter a short list of possible elements, otherwise the generated hit list will be too large and the calculation time too long even with the most powerful computers. The accurate mass method will not provide any information if the molecular ion does not appear in the mass spectrum and may provide false identification of a fragment or impure ion.